Organisms from bacteria to humans use a circadian clock to control daily biochemical, physiological, and behavioral rhythms. This clock affects human physiology, and disruptions of normal clock function can cause a variety of health problems. A considerable amount is known about the oscillators that form the core of the circadian timing system. However, the output pathways that connect oscillators to the activities they control are largely unknown. In humans, p38 mitogen-activated protein kinase (MAPK) pathways regulate cell proliferation, and mutations of pathway components, or alterations of pathway activity, are associated with cancer. We found that in Neurospora crassa the stress-induced p38 MAPK pathway is used as an output pathway from the FRQ/WCC circadian oscillator. Under non-stressed conditions, time-of-day information is passed from the oscillator through the MAPK signaling pathway at or upstream of the response regulator RRG-1, resulting in rhythmic p38 MAPK phosphorylation. Phospho-p38 MAPK then signals to transcription factors and other effector molecules, to regulate rhythms in gene expression. We hypothesize that circadian oscillators have co-opted cellular signaling pathways to control expression of clock-controlled genes (ccgs), and propose three specific aims to test this hypothesis. First, to elucidate the mechanism whereby ccgs are regulated by the clock, we will use available mutants to determine which p38 MAPK pathway components are necessary for clock regulation of the pathway. We will also determine which oscillator components are needed for this regulation. Secondly, again using available mutants, we will identify the transcription factor(s) that function downstream of the p38 MAPK to regulate rhythmic expression of ccg-1, a gene regulated by the p38 MAPK pathway. These results will allow us to describe, for the first time in any organism, a complete mechanism for rhythmic regulation of a ccg. Thirdly, to investigate whether co-opting cellular signaling pathways by the clock is a general mechanism for controlling rhythmic gene expression, we will determine if the clock regulates only p38, or p38 and other MAPK signaling pathways in Neurospora. Lastly, we will examine whether co-opting cellular signaling pathways by the clock is a conserved mechanism for controlling circadian rhythmicity by determining if the clock also regulates rhythmicity of the mammalian p38 pathway. Because the p38 kinases are known to regulate chromatin structure and translation, we will also investigate the exciting possibility that rhythmic p38 MAPK activity contributes to circadian rhythmicity by regulating chromatin remodeling and translational repression. Together, results from these aims will uncover the mechanisms by which circadian oscillators signal through the output pathways to control gene expression and may provide novel approaches for therapies to treat human diseases that result from circadian dysfunction and/or misregulation of the p38 MAPK pathway. PUBLIC HEALTH RELEVANCE Humans have an internal clock that regulates mechanisms such as the sleep/wake cycle and affects the response of cells to anti-cancer drugs and other pharmaceuticals. We found that the circadian clock in fungi regulates daily rhythms in the activity of p38 mitogen-activated protein kinase, which functions in mammals to suppress cancer cell formation. Understanding how the clock regulates the p38 pathway will facilitate efforts for developing new therapies for treating human diseases that result from clock dysfunction and/or misregulation of the p38 MAPK pathway.